Cell synchronization is the very first step when UE wants to camp on any cell, through which UE acquires physical cell identification (PCID), time slot and frame synchronization. Such parameters enable UE to read system information blocks from a particular network. During cell synchronization, UE tunes its radio by turning to different frequency channels depending upon which bands it is supporting. Assuming that it is currently tuned to a specific band/channel, FDD UE first finds a primary synchronization signal (PSS) located in the last OFDM symbol of the first time slot of the first subframe (subframe 0) of a radio frame as shown in FIG. 1. PSS enables UE to be synchronized on subframe level. PSS is repeated in subframe 5, which means that UE is synchronized on a 5-ms basis since each subframe is 1-ms. Next, FDD UE finds a secondary synchronization signal (SSS). As shown in FIG. 1, SSS symbols are also located in the same subframe where PSS is found but within the symbol immediately before PSS. Once UE knows the PCID for a given cell, it also knows the location of cell reference signals, which are used in channel estimation, equalization, cell selection/reselection and handover procedures. For TDD UE, the synchronization process is similar to the above-mentioned procedure except that PSS is located at the third OFDM symbol in subframes 1 and 6, and SSS is located at the last OFDM symbol in subframes 0 and 5.
The primary and secondary synchronization signals are defined in LTE to serve UE's initial synchronization and cell search. Both PSS and SSS occupy the central 72 subcarriers within system bandwidth, and are both transmitted once per 5-ms or 5 subframes. Note that the occupied 72 subcarriers include 10 reserved (unused) subcarriers that are evenly located at the two ends of 72 subcarriers. The number of subcarriers containing active PSS/SSS signal is 72−10=62.
In wireless communication system, the downlink reference signals are set up to provide reference to a mobile station (also called “UE”) for channel estimation used in coherent demodulation of data channels. In LTE Rel-8 specification, this type of downlink reference signal is called cell-specific reference signal (CRS). CRS is used by UE to compensate the downlink frame for channel frequency response and cross-channel effects so that the signal can be demodulated. With up to four ports (ports 0˜3) on CRS, the base station (also called “cell” or “eNB”) needs to inform UE of precoding matrix being used in MIMO transmission on physical downlink shared channel (PDSCH). In LTE Rel-10 specification, UE-specific demodulation reference signal (DMRS) is defined on up to eight ports (ports 7˜14) to provide transparent precoding in MIMO transmission on PDSCH. Both CRS and DMRS are defined on logical antenna ports, and the precoding on PDSCH and the accompanied demodulation reference signals map transmission from the logical antenna ports to physical antenna ports. Starting from LTE Rel-11, DMRS is also used as demodulation reference signal for enhanced physical downlink control channel (EPDCCH). EPDCCH has a single layer transmission, but may use any ports from logical antenna ports {107,108,109,110}. The resource allocation patterns for both Rel-8 CRS and Rel-10&11 DMRS are defined per physical resource block (PRB) pair, as shown in FIG. 2. In one normal cyclic prefix (CP) configuration, one PRB pair contains 12×14 resource elements (RE) in 2-D time-frequency resource area across over 12 subcarriers and 14 OFDM symbols. The time duration of one PRB pair is 1-ms, which is also called one subframe in LTE. Ten continuous subframes, indexed 0˜9, constructs one radio frame.
As shown in FIGS. 1 and 2, PSS/SSS and DMRS may map to the same resource element (RE), which causes a resource element collision. Up to LTE Rel-11, such collision is not a big problem, because it can be avoided by implementations at eNB. In the PRB pair containing PSS/SSS, eNB can schedule PDSCH using CRS as demodulation reference and therefore avoid transmitting DMRS. However, when it comes to LTE Rel-12, new carrier type (NCT) is discussed. On the NCT carrier, CRS is no longer used as demodulation reference and it is transmitted on single port (port 0) once per 5-ms to provide reference for time tracking. Therefore, only UE-specific DMRS is used as demodulation reference for both PDSCH and EPDCCH on NCT. Then the collision of PSS/SSS REs and DMRS REs in the PRB pair containing PSS/SSS becomes unavoidable. Because PSS/SSS is the common cell-specific signal, it has to be transmitted, which then blocks DMRS transmission on the colliding REs. This may result a significant degrade in PDSCH/EPDCCH performance especially when UE speed is not low.